I've been down to JNB (obviously not CPT) a few times - I've always found the LON-JNB route as one of the most expensive routes globally although admittedly haven't done detailed research into it. I'd guess CPT isn't too much different especially during the Northern Hemisphere winter.
Booked just a few days before going indirect via DXB, which is 16 hours in the air, I've spent over £5k in business return, although shopping around is important.